function animate(obj,target,callback){  
    clearInterval(obj.timer)     
    obj.timer =setInterval(function(){
           var step =(target - obj.offsetLeft)/10;
           step = step > 0 ? Math.ceil(step) : Math.floor(step);
           if(obj.offsetLeft==target){
                 clearInterval(obj.timer); 
                 if(callback){    
                          callback();
                 }
           
   }
   obj.style.left =obj.offsetLeft+step+'px';  
},30)
}